PulseAudio

PulseAudio

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PulseAudio

PulseAudio (formerly PolypAudio) is a cross-platform, networked sound server project. It is intended to be an improved drop-in replacement for the Enlightened Sound Daemon (ESD).

PulseAudio runs under Microsoft Windows and POSIX-compliant systems like Linux. Released under the terms of the GNU Lesser General Public License (for the software library portion) and the GNU General Public License (for the sound server itself), PulseAudio is free software.

Features
Operation
Adoption
Alternatives
See also
References
External links

Features

The main PulseAudio features include:

Per-application volume controls^[1]
An extensible plugin architecture with support for loadable modules
Compatibility with many popular audio applications
Support for multiple audio sources and sinks
Low-latency operation and support for latency measurement
A zero-copy memory architecture for processor resource efficiency
Ability to discover other computers using PulseAudio on the local network and play sound through their speakers directly
Ability to change which output device an application plays sound through while the application is playing sound (without the application needing to support this, and indeed without even being aware that this happened)
A command-line interface with scripting capabilities
A sound daemon with command line reconfiguration capabilities
Built-in sample conversion and resampling capabilities
The ability to combine multiple sound cards into one
The ability to synchronize multiple playback streams
Bluetooth audio devices with dynamic detection

Operation

PulseAudio operational flow chart

PulseAudio is a sound server, a background process accepting sound input from one or more sources (processes or capture devices) and redirecting it to one or more sinks (sound cards, remote network PulseAudio servers, or other processes).

One of the goals of PulseAudio is to reroute all sound streams through it, including those from processes that attempt to directly access the hardware (like legacy OSS applications). PulseAudio achieves this by providing adapters to applications using other audio systems, like aRts and ESD.

In a typical installation scenario under Linux, the user configures ALSA to use a virtual device provided by PulseAudio. Thus, applications using ALSA will output sound to PulseAudio, which then uses ALSA itself to access the real sound card. PulseAudio also provides its own native interface to applications that want to support PulseAudio directly, as well as a legacy interface for ESD applications, making it suitable as a drop-in replacement for ESD.

For OSS applications, PulseAudio provides the padsp utility, which substitutes device files such as /dev/dsp, tricking the applications into believing that they have exclusive control over the sound card. In reality, their output is rerouted through PulseAudio.

Adoption

Linux distributions such as Ubuntu 8.04 and Fedora 8 introduced PulseAudio as the default audio system arguably before it was ready: PulseAudio developer Lennart Poettering described it as "the software that currently breaks your audio" ^[2]. He later claimed that "Ubuntu didn't exactly do a stellar job---they didn't do their homework" in adopting PulseAudio.^[3]

Alternatives

ALSA provides a software mixer called dmix, which was developed prior to PulseAudio, imposes less overhead, and is enabled by default on modern systems. However, it does not provide any of the advanced features of PulseAudio, such as re-sampling, aggregating multiple sound cards into one and network audio. However, these abilities are not typically used by common users.

PulseAudio can also interoperate with legacy sound systems, including those that were designed to exclusively lock the sound card. This achieves more complete redirection of audio streams into the mixer.